The third group of macromolecules found in cells contain some of the largest molecules of the body - these are the nucleic acids. There are two major groups of nucleic acids, ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Each group is composed of long chains of building block molecules known as nucleotides. There are four different nucleotides used in the formation of nucleic acids, these are adenine, thymine (uracil instead of thymine is found in RNA), guanine and cytosine (see Figure 11). Each nucleotide contains three distinct parts: a 5-carbon sugar (ribose in RNA, deoxyribose in DNA), at least one phosphate group attached to the number 5 carbon of the sugar, and one of four different nitrogenous bases that attach to the number 1 carbon of the sugar. Nucleotides are bound together via a phosphodiester bond between the phosphate group of one nucleotide and the number 3 carbon of another. In this manner, very long chains (over 1 million nucleotides long) are formed.
The primary function of DNA is as the carrier of genetic information. DNA is found only in the nucleus of the cell and is the genetic component of the chromosomes. DNA is usually double stranded, meaning that two strands of nucleotides are attached to one another via H-bonds between different bases. Since each base can only form H-bonds with only one type of base (adenine binds with thymine, cytosine with guanine), the sequence of bases in one strand must complement the sequence of the other (and therefore, if you know the sequence in one strand, you can deduce the sequence of the other). During cell replication, the two strands separate and each is used as a template for a new strand. In this manner, one double stranded DNA molecule is perfectly replicated into two identical strands, one for each daughter cell. DNA is made only from preexisting DNA (any exceptions do not concern us in this course).
RNA is involved in synthesis of all proteins by the cell. As you should know, proteins are synthesized on cytoplasmic organelles known as ribosomes. Also, in order to make a particular protein, you need to know the sequence of amino acids. DNA contains codes for the amino acid sequence for all proteins made by the cell. However, DNA doesn't leave the nucleus and ribosomes don't enter the nucleus. So there is a problem, how does the code for a protein (that's in the DNA) get to the ribosome? The answer is through RNA.
Let us suppose that a cell needs to make a particular protein, say myoglobin. Somewhere on the DNA in the nucleus is a small stretch of nucleotides that code for the sequence of myoglobin. Through mechanisms that don't concern us yet, the nucleus activates enzymes that cause a complementaly strand RNA to be synthesized using the area of DNA containing the code for myoglobin as a template. In this manner, a strand of mRNA (for messenger RNA) is synthesized (a process known as transcription) that contains the code for the amino acid sequence of myoglobin. The mRNA then leaves the nucleus, travels to the ribosome where the code is interpreted and the protein synthesized (a process known as translation). This process occurs for the synthesis of all proteins and is summarized by the following:
There are two other functions of nucleotides that will be important to physiology. One is as a second messenger molecule which will be describe later in the semester. The other is the use of adenosine triphosphate (ATP, a nucleotide) as the energy currency of the cell. ATP can be hydrolyzed as shown in equation 8 with the release of energy. It is the energy derived from this reaction that the cell uses to drive almost every cellular function or reaction that requires energy. Any enzyme that catalyzes this reaction is known as an ATPase (there are many different ATPases in cells). New ATP is synthesized by the reverse of its hydrolysis using energy that comes from the metabolism of nutrients via glycolysis and the TCA cycle.
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